INTERPLAY BETWEEN EOLIAN SEDIMENTATION, WATER TABLE DYNAMICS, WET-DRY CLIMATE, THERAPOD DINOSAURS, AND PROTOMAMMALS PRESERVED IN THE LOWER JURASSIC AZTEC SANDSTONE, VALLEY OF FIRE STATE PARK, SOUTHERN NEVADA

An anomalous interbedded succession of reddish-brown mudstone, sandstone, and light green dolomitic limestone crops-out as a resistant ledge among predominantly quartzose eolian cross-beds in the Lower Jurassic Aztec Sandstone near Mouse’s Tank at Valley of Fire State Park, southern Nevada. From a maximum thickness of 3.0 meters, the bed thins out to the east and possibly to the west, and is typified by localized sandy cross-bed sets 0.1 m thick, erosional scours a few centimeters deep, and broken, folded, wavy, and flat mudstone and very fine grained sandstone laminations. In bedding cycles, 0.8 to 1.0 meter thick, are platy calcareous fragments that increase upward in size and abundance in each cycle. Trace fossils have not been found stratigraphically below this interval. However, vertebrate tracks are found in strata within a few meters above this interval.

We interpret this bedding succession to represent deposition in a localized interdune swale now preserved in the eolian Aztec Sandstone. In response to wetter climate or subsidence of the sediment pile, the water table rose and intersected a dry, nearly lifeless desert surface. A desert ecosystem then sprang to life, with a diverse biota of plants, not preserved as fossils, and arthropods, therapsids, and carnivorous theropod dinosaurs, preserved as trackways. Limited areal extent of the interdune interval suggests a short-lived, wet ecosystem that eventually succumbed to drying conditions, lowered water table, more sediment available for wind transport, and burial by eolian dunes. Laminations of fine wind-blown sand accumulated by capillary trapping near the top of the water table. Occasional wadi floods transported small subaqueous dunes and scoured the interdune flat. Mudstone laminations settled from suspension in standing water and waning wadi flooding, and were later deformed by loading. Platy calcareous fragments may be remnants of organic mats that record vertical fluctuations in the water table in upward-wetting cycles.